1. Field of the Invention
The present invention relates to a zoom lens.
2. Related Background Art
In recent years, zoom lens is principally adopted as the phototaking lens for the camera with lens shutter.
With the improvement in the performance of zoom lens, there have been proposed zoom lenses of various types. Also with the recent progress in the barrel technology, there have been proposed so-called multi-group zoom lenses achieving a larger zoom ratio, composed of three or more movable lens groups. Along with such trend, there have also been proposed various focusing systems for such multi-group zoom lenses.
At first there will be given general explanation on so-called multi-group zoom lens, having three or more movable lens groups.
In such zoom lens with multiple lens groups, the zooming trajectories of the lens groups involved in the refractive power varying operation from the wide angle end to the telescopic end have a larger freedom of selection, so that the freedom for correction of aberrations also increases. Also the increase in the number of lens groups involved in the refractive power varying operation allows to uniformly distribute the burden of such refractive power variation among the lens groups, so that it becomes possible to attain higher performance while achieving a larger zooming ratio. Such zoom lens has been associated with a difficulty of complication in the barrel structure resulting from the increase in the number of movable parts, but such difficulty has been overcome to a certain extent by the recent progress in the barrel technology.
In the zoom lens without limitation in the back focus, it has been customary to employ a negative lens group at the position closest to the image side in order to reduce the length of the entire lens system and to reduce the diameters of the component lenses, and, in such lens, the refractive power varying operation, from the wide angle end to the telescopic end, has been effectively achieved by increasing the change in the back focus.
It is desirable also to employ a positive lens group closest, in the entire lens system, to the object side in order to reduce the length of the entire lens system at the telescopic end, and to position said positive lens group as closely as possible to the film plane at the wide angle end, in order to reduce the effective diameter of the frontmost lens component.
Based on these facts, a positive-positive-negative three-group zoom lens and a positive-negative-positive-negative four-group zoom lens have been raised as the specific configurations of such multigroup zoom lens adapted for compactization and capable of achieving a large zooming ratio, and various proposals have been made on these configurations.
The positive-positive-negative three-group zoom lens is composed, in the order from the object side, of a first lens group having a positive refractive power, a second lens group having a positive refractive power and a third lens group having a negative refractive power, and is so constructed that, at the refractive power varying operation from the wide angle end to the telescopic end, the air gap between said second and third lens groups decreases while the air gap between said first and second lens group increases. The positive-positive-negative three-group zoom lens of such configuration is disclosed, for example, in the Japanese Patent Application Laid-open No. 2-256015.
In the zoom lens disclosed in the above-mentioned patent application, a major portion of the effect of the refractive power variation is borne by the third lens group. For this reason, in a design for a larger zooming ratio, there will result a large variation in the off-axial aberrations generated in the third lens group, at the refractive power varying operation from the wide angle end to the telescopic end.
On the other hand, the second lens group is composed of a lens group 2a having a negative refractive power at the object side and a lens group 2b having a positive refractive power at the image side. Consequently the above-mentioned variation in the aberrations resulting from the refractive power varying operation can be suppressed to a certain extent, either by increasing the number of lens components constituting the second lens group or by selecting a sufficiently large axial distance between said lens groups 2a and 2b. Such solutions, however, are difficult to achieve the reduction in the number of component lenses or the simplification of the configuration, and are generally against compactization.
The positive-negative-positive-negative four-group zoom lens is composed, as disclosed for example in the Japanese Patent Application Laid-open No. 60-57814, in the order from the object side, of a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power and a fourth lens group having a negative refractive power, and is so constructed that, at the refractive power varying operation from the wide angle end to the telescopic end, the air gap between said first and second lens groups increases while that between said third and fourth lens group decreases.
In the zoom lens disclosed in the above-mentioned patent application, the length of the entire lens system at the telescopic end is reduced by selecting the refractive power of the first lens group larger, in the positive side, than the refractive power of the entire lens system at the telescopic end, and selecting the second to fourth lens groups in such a manner that the synthesized refractive power thereof becomes negative at the telescopic end.
However, in the above-mentioned zoom lens, the second lens group has, at the wide angle end, a magnification .beta.2 at use, approaching a relation 1/.beta.2.congruent.0, indicating that the synthesized refractive force of the first and second lens groups is almost zero. For this reason, in case of designing for a wider image angle, there cannot be obtained a sufficiently large back focus and the effective diameter of the rear lens components becomes inevitably larger.
Also the above-mentioned zoom lens can achieve a larger zoom ratio, in comparison with the positive-positive-negative three-group zoom lens, by making a large change in the air gap between the second and third lens groups at the refractive power varying operation from the wide angle end to the telescopic end, thereby causing said first to third lens groups to participate in the refractive power varying operation. However, if the compactization of the lens system is intended at the same time, it becomes difficult to satisfactorily suppress the variation in the off-axial aberrations generated in the second lens group, because the off-axial rays passing said second lens group does not vary much in the height but varies significantly in the incident angle at the refractive power varying operation.
As explained in the foregoing, in the conventional positive-positive-negative three-group zoom lens or positive-negative-positive-negative four-group zoom lens, it has been difficult to achieve higher performance in the imaging characteristics while attaining compactization and a larger zoom ratio. Also in case of designing for a larger zoom ratio with a limited number of component lenses, each of the lens groups has a larger burden in the refractive power varying operation, so that it has been difficult to satisfactorily correct the variations in the aberrations, generated in the refractive power varying operation from the wide angle end to the telescopic end.
In the following there will be explained the focusing method in the multigroup zoom lens.
In general, the focusing lens group is required to have a smaller amount of movement and a smaller weight, because a smaller amount of movement can lead to the compactization of the entire lens system, while a smaller lens weight allows to simplify the lens driving mechanism.
For focusing to a short-distance object in the multigroup zoom lens, various proposals have been made in the following three methods:
(A) single lens group moving method;
(B) IF (inner focusing) method;
(C) RF (rear focusing) method.
In the multigroup zoom lens without limitation in the back focus, as represented by the conventional positive-positive-negative three-group zoom lens or positive-negative-positive-negative four-group zoom lens, a positive lens group is provided closest to the object side within the lens system (from a lens face closest to the object side to a lens closest to the image side), but said positive lens group is given a weak positive refractive power in order to satisfactorily correct the positive distortion generated at the wide angle end. Consequently the amount of movement becomes large in case of focusing with the method (A) by single lens movement.
Also as the back focus at the wide angle end is relatively small, the negative lens group closest to the image side has a lens diameter larger than that in other lens groups and bears a large proportion in the refractive power varying operation. For this reason, said negative lens group requires at least two or more lenses, so that the focusing lens group becomes heavy in case of focusing with the rear focusing method (C).
The inner focusing method (B), in case of the positive-positive-negative three-group zoom lens, is achieved, as disclosed in the Japanese Patent Application Laid-open No. 4-338910, by dividing the second lens group into two and moving a lens group at the object side toward the object side. However, the focusing group becomes undesirably heavy because the focusing group contains an increased number of lenses.
In case of the positive-negative-positive-negative four-group zoom lens, the second lens group shows a large variation in the magnification of use at the refractive power varying operation, as disclosed for example in the Japanese Patent Application Laid-open No. 3-39920. For this reason, the number of component lenses has to be increased in order to suppress the variation in the aberrations at the focusing operation, while suppressing also the variation in the aberration at the refractive power varying operation, and the focusing group therefor becomes undesirably heavy.
Besides, the conventional multigroup zoom lenses have not been satisfactory in reducing the amount of work (weight.times.amount of movement) at the focusing operation.